Flashy, your a pip, good to hear from you! By mid 1885 troops were wearing it in the field, in particular the Southwest against the Apache. This is why Leonard Wood specified the 1884 for the 1st USV for field wear. There are several photos of troops in Tampa 1898 wearing them, or just the trousers with the blue campaign shirt. I even have some Edison Video of regular troops marching to transports, wearing 1884s. Its hard to tell, but as the trousers and tunics appear the same shade, and the leggings the same shade as well, I believe this to be the case, as the Brown duck 1884s were the only outfit that matched the leggings. What we do not have is studio photos of this, as troops wanted to be seen in their more stylish "BLues".
I also have a photo of regulars in front of a damaged spanish fortification in the Phillipines, wearing 1884 trousers and blue shirts, rifles in hand.
In any case, we have much documented photos and footage of the 1st USV, Including Theodore Roosevelts letters to his Harvard chums, advising them to get 1884s to match the troops, as the Khakis would never be ready in time.
I believe the 1884 was the most common field uniform in Cuba in 1898, but not often pictured, as the New York Press wanted photo's of the 71st NY, a NG unit forced to the front by their Senator, wearing the blue wool uniform. But the Regular army was not as dumb as we like to think: why wouldn't they use the most readily available tropical weight uniform, one that every regular soldier already had in his kit?
Vol 1, Chapter 5 of Douglas McChristians "Uniforms, Arms and Equipment, 1880-1892" Spends quite a bit of time on the 1884 Fatigues, and it becomes quite clear that the men liked it, and that local officers quickly sent it into the field. It also becomes quite clear that this was NOT intended to be worn over the existing wool uniform, but was to be worn "Similiar to the working clothes worn by the miners in the Western states and territories" This is a quote of Quartermaster General Samuel Holabird, the originator of the 1884.
The 1884 was authorized by General order No 32, April 16 1884. It was stipulated that the garments were to be worn on all work and fatigue details EXCEPT Stable duty, to preserve the Wool uniform. The older White Stable Frock was still to be issued to Cavalrymen for that. THAT would be the garment worn over the existing uniform, including the 1884! Officers were also authorized to draw this canvas clothing as well! Many officers interpreted the above order to include field duty as well, saving the wool for dress occasions.